A ksatriya cannot tolerate insults on the principle of fighting. Bhismadeva therefore promised that the next day he would kill all five Pandavas with special weapons made for the purpose
SB Canto 1
Fulfilling my vow and sacrificing His own promise, He got down from the chariot, took up its wheel, and ran towards me hurriedly, just as a lion goes to kill an elephant. He even dropped His outer garment on the way.
The Battle of Kurukṣetra was fought on military principles but at the same time in a sporting spirit, like a friend's fight with another friend. Duryodhana criticized Bhīṣmadeva, alleging that he was reluctant to kill Arjuna because of paternal affection. A kṣatriya cannot tolerate insults on the principle of fighting. Bhīṣmadeva therefore promised that the next day he would kill all five Pāṇḍavas with special weapons made for the purpose. Duryodhana was satisfied, and he kept the arrows with him to be delivered the next day during the fight. By tricks Arjuna took the arrows from Duryodhana, and Bhīṣmadeva could understand that this was the trick of Lord Kṛṣṇa. So he took a vow that the next day Kṛṣṇa would have to take up weapons Himself, otherwise His friend Arjuna would die. In the next day's fighting Bhīṣmadeva fought so violently that both Arjuna and Kṛṣṇa were in trouble. Arjuna was almost defeated; the situation was so tense that he was about to be killed by Bhīṣmadeva the very next moment. At that time Lord Kṛṣṇa wanted to please His devotee, Bhīṣma, by keeping Bhīṣma's promise, which was more important than His own. Seemingly He broke His own promise. He promised before the beginning of the Battle of Kurukṣetra that He would remain without weapons and would not use His strength for either of the parties. But to protect Arjuna He got down from the chariot, took up the wheel of the chariot and hurriedly rushed at Bhīṣmadeva in an angry mood, as a lion goes to kill an elephant. He dropped His covering cloth on the way, and out of great anger He did not know that He had dropped it. Bhīṣmadeva at once gave up his weapons and stood to be killed by Kṛṣṇa, his beloved Lord. The fighting of the day was thus ended at that very moment, and Arjuna was saved. Of course there was no possibility of Arjuna's death because the Lord Himself was on the chariot, but because Bhīṣmadeva wanted to see Lord Kṛṣṇa take up some weapon to save His friend, the Lord created this situation, making Arjuna's death imminent. He stood before Bhīṣmadeva to show him that his promise was fulfilled and that He had taken up the wheel.